Including rebuilds - i've gone through 5 alternators in 7 years, the last one was rebuilt twice, so 3 physical alternators.
in my case it has always been the the front bearings failing or the diode pack on the back dying. Luckily the brush and diode pack on the back of the alt is easily replacable on mine - no idea what the alts are like for servicability on a p38 engine though. I just accept that an alternator is service item now and when the red light flickers after a good dunking i just ignore it until i get home and hose it out. Usually the red light goes away after a day or two drying out. If it doesnt, then i'll have a proper poke about.
Water on its own isnt usually the killer - its gritty muddy water that does the damage, so the sooner you can clean it out the better.

hey chaps,I get them exchange...There is a smart charge system on the P38 which begins to shut things down as the power gets low,It started getting rather noisy ...then while driving the battery light came on but on tickover went out....so was charging on tickover...but then it failed altogether,so i managed to limp it home (without having my pie and peas from the trial)...and then i dumped it on the drive.......bloomin thing.
Now am in the luxury of the V8 and rapidly losing interest in the diesel..